Human cytochrome P-450IIE1 has been implicated in the oxidation of a number of substrates, including protoxins and -carcinogens. To date, no drugs have been identified that are exclusive substrates for the protein and are applicable for use as noninvasive probes of the in vivo function of the enzyme in humans. Chlorzoxazone was found to be oxidized only to 6-hydroxychlorzoxazone in human liver microsomes. Results of steady-state kinetics are consistent with the view that only a single enzyme catalyzes the reaction. The microsomal reaction was strongly inhibited by rabbit anti-P-450IIE1 and, in a competitive manner, by known P-450IIE1 substrates. Rates of chlorzoxazone 6-hydroxylation in different human liver microsomal preparations were well correlated with levels of immunochemically measured P-450IIE1 and rates of (CH3)2NNO oxidation. Chlorzoxazone 6-hydroxylation was also found to be catalyzed by purified human liver P-450IIE1. These results provide strong evidence that P-450IIE1 is the primary catalyst of chlorzoxazone 6-hydroxylation in human liver. Rates of chlorzoxazone 6-hydroxylation vary considerably among human liver samples, and chlorzoxazone 6-hydroxylation may have potential use as a noninvasive probe in estimating the in vivo expression of human P-450IIE1 and its significance as a risk factor in the toxicity and carcinogenicity of a number of solvents, nitrosamines, and drugs.